White, biaxially oriented polyester film with cycloolefin copolymer (COC), process for producing the film, and its use

ABSTRACT

The present invention relates to a white, biaxially oriented polyester film having at least a base layer, the characterizing features of which are that at least this base layer comprises a cycloolefin copolymer (COC) at a concentration of from 4 to 60% by weight, based on the base layer, where the glass transition temperature of cycloolefin copolymer (COC) is within the range from 70 to 270° C.

The present invention relates to a white, biaxially oriented polyesterfilm which comprises at least one layer comprising a polyester and acycloolefin copolymer (COC). The invention also relates to a process forproducing the polyester film, and also to the use of the film.

BACKGROUND OF THE INVENTION

White, biaxially oriented polyester films are known in the prior art.These films known in the prior art are either easy to produce, have goodoptical properties or have acceptable processing performance.

DE-A 2 353 347 describes a process for producing a milky polyester filmhaving one or more layers, which comprises preparing a mixture ofparticles of a linear polyester with from 3 to 27% by weight of ahomopolymer or copolymer of ethylene or propylene, extruding the mixtureas a film, quenching the film and biaxially orienting the same indirections running perpendicularly to one another, and heat-setting thefilm. A disadvantage of the process is that regenerated materialproduced during the production of the film (essentially a mixture ofpolyester and ethylene or propylene copolymer) cannot be reused withoutyellowing of the film. However, this makes the process uneconomical, andthe film produced with regenerated material was not successful on themarket. In addition, the film has roughness values which aresignificantly too high, giving it a very matte appearance (very lowgloss) that is undesirable for many applications.

EP-A 0 300 060 describes a single-layer polyester film which comprises,besides polyethylene terephthalate, from 3 to 40% by weight of acrystalline propylene polymer and from 0.001 to 3% by weight of asurface-active substance. The effect of the surface-active substance isto increase the number of vacuoles in the film and at the same time toreduce their size to the extent desired. This achieves higher opacityand lower density of the film. A disadvantage of the film continues tobe that regenerated material produced during the production of the film(essentially a mixture of polyester and propylene homopolymer) cannot bereused without yellowing of the film. However, this makes the processuneconomical, and the film produced with regenerated material was notsuccessful on the market. In addition, the film has roughness valueswhich are significantly too high, giving it a very matte appearance(very low gloss) that is undesirable for many applications.

EP-A 0 360 201 describes a polyester film having at least two layers,comprising a base layer with fine vacuoles, the density of which is from0.4 to 1.3 kg/dm³, and at least one outer layer, the density of which isgreater than 1.3 kg/dm³. The vacuoles are achieved by adding from 4 to30% by weight of a crystalline propylene polymer, followed by biaxialstretching of the film. As a result of adding the outer layer the easeof production of the film is improved (no streaking on the filmsurface), the surface tension is increased and the roughness of thelaminated surface can be reduced. A disadvantage still present is thatregenerated material produced during the production of the film(essentially a mixture of polyester and propylene homopolymer) cannot bereused without yellowing of the film. However, this makes the processuneconomic, and the film produced with regenerated material was notsuccessful on the market. In addition, the film listed in the examplescontinues to have excessive roughness values, giving it a matteappearance (low gloss) that is undesirable for many applications.

EP-A 0 795 399 describes a polyester film having at least two layers andcomprising a base layer with fine vacuoles, the density of which is from0.4 to 1.3 kg/dm³, and having at least one outer layer, the density ofwhich is greater than 1.3 kg/dm³. The vacuoles are achieved by addingfrom 5 to 45% by weight of a thermoplastic polymer to the polyester inthe base, followed by biaxial stretching of the film. The thermoplasticpolymers used are, inter alia, polypropylene, polyethylene,polymethylpentene, polystyrene or polycarbonate, and the preferredthermoplastic polymer is polypropylene. As a result of adding the outerlayer the ease of production of the film is improved (no streaking onthe film surface), the surface tension is increased and the roughness ofthe laminated surface can be matched to prevailing requirements. Furthermodification of the film in the base layer and/or in the outer layers,using white pigments (generally TiO₂) and/or using optical brightenerspermits the properties of the film to be matched to the prevailingrequirements of the application. A continuing disadvantage is thatregenerated material produced during the production of the film(essentially a mixture of polyester and the added polymer) cannot bereused without undefined and undesirable changes in the color of thefilm. This makes the process uneconomical, and the film produced withregenerated material was not successful on the market. In addition, thefilms listed in the examples continue to have excessive roughnessvalues, giving it a matte appearance (low gloss) that is undesirable formany applications.

DE-A 195 40 277 describes a polyester film having one or more layers,which comprises a base layer with fine vacuoles, the density of which isfrom 0.6 to 1.3 kg/dm³, and which has a planar birefringence of from−0.02 to 0.04. The vacuoles are achieved by adding from 3 to 40% byweight of a thermoplastic resin to the polyester in the base, followedby biaxial stretching of the film. The thermoplastic resins used are,inter alia, polypropylene, polyethylene, polymethylpentene, cyclicolefin polymers, polyacrylic resins, polystyrene or polycarbonate, andpreferred polymers are polypropylene and polystyrene. Maintaining thestated limits for the birefringence of the film gives the claimed filmin particular a superior tear strength and superior isotropicproperties. A continuing disadvantage is that regenerated materialproduced during the production of the film cannot be reused withoutundefined and undesirable changes in the color of the film. This makesthe process uneconomical, and the film produced with regeneratedmaterial was not successful on the market. In addition, the films listedin the examples continue to have excessive roughness values, giving it amatte appearance (low gloss) that is undesirable for many applications.

EP-A 0 300 060, EP-A 0 360 201, EP-A 0 795 399, and DE-A 195 40 277 areincorporated by reference for their teaching with respect to theinitiation of vacuoles in film layers.

DESCRIPTION OF THE INVENTION

The object of the present invention was to provide a white, biaxiallyoriented polyester film which has high gloss, and in particular iseasier to produce, i.e. has lower production costs. In particular itshould be ensured that the regenerated material produced directly duringthe production process can be reused at a concentration of from about 10to 70% by weight, based on the total weight of the film, without anysignificant negative effect on the physical properties of the film. Inparticular, adding regenerated material should not give any significantyellowing of the film.

The object has been achieved by means of a white, biaxially orientedpolyester film having at least a base layer, the characterizing featuresof which are that at least this base layer additionally comprises acycloolefin copolymer (COC) at a concentration from about 4 to 60% byweight, based on the base layer, where the glass transition temperatureof the cycloolefin copolymer (COC) is in the range from about 70 to 270°C.

For the purposes of the present invention, a white, biaxially orientedpolyester film is a film of this type which has a whiteness above about70%, preferably above about 75%, particularly preferably above about80%. The opacity of the novel film is moreover above about 55%,preferably above about 60% and particularly preferably above about 65%.

To achieve the desired whiteness of the novel film the proportion of thecycloolefin copolymer (COC) in the base layer must be greater than about4%, otherwise the whiteness is below about 70%. If, on the other hand,the cycloolefin copolymer (COC) content is greater than about 60%, theproduction of the film becomes uneconomic, since the orientation processbecomes unreliable.

It is also necessary for the glass transition temperature of thecycloolefin copolymer (COC) used to be above about 70° C. If this is notthe case (if the glass transition temperature is below about 70° C.) thepolymer mixture is difficult to process (difficult to extrude), thedesired whiteness is no longer achieved and the regenerated materialused gives a film with a tendency toward increased yellowing. If, on theother hand, the glass transition temperature of the selected cycloolefincopolymer (COC) is above about 270° C., the polymer mixture will nolonger be capable of sufficiently homogeneous dispersion in theextruder. The result is a film with non-uniform properties.

In the preferred embodiment of the novel film the glass transitiontemperature of the COCs used is in the range from about 90 to 250° C.,and in the particularly preferred embodiment it is in the range fromabout 110 to 220° C.

Surprisingly, it has been found that adding a cycloolefin copolymer(COC) in the manner described above can produce a white, opaque andglossy film.

The whiteness and the opacity of the film can be precisely controlledand matched to the prevailing requirements as a function of the amountand type of the cycloolefin copolymer (COC) added. It is possible bythis means to dispense to a considerable extent with the use of otherwhiteners and opacifiers. It was also highly surprising that the surfaceroughness of the film is significantly lower, and therefore the gloss ofthe film is significantly higher, than in comparable films of the priorart. The additional effect—that the regenerated material, unlike thepolymeric additives of the prior art, shows no tendency to causeyellowing—was entirely surprising.

None of these features described was foreseeable, especially sincealthough it is clear that COCs are substantially incompatible withpolyethylene terephthalate, it is known that they are oriented withsimilar stretching ratios and stretching temperatures to those used forpolyethylene terephthalate. In these circumstances the skilled workerwould have expected that a white and opaque film with a high glosscannot be produced under these process conditions.

In the preferred and particularly preferred embodiments the novel filmhas high and, respectively, particularly high whiteness and high and,respectively, particularly high opacity, and the change in color of thefilm as a result of adding regenerated material remains extremely small.

The novel film has one or more layers. The structure of single-layerembodiments is as for the COC-containing layer described below.Embodiments having more than one layer have at least two layers andalways comprise the COC-containing layer and at least one other layer,where the COC-containing layer is the base layer, but may additionallyalso form the intermediate layer or the outer layer of the film havingmore than one layer. In a preferred embodiment the COC-containing layerforms the base layer of the film with at least one outer layer andpreferably with outer layers on both sides, and (an) intermediatelayer(s) may, if desired, be present on one or both sides. In anotherpreferred embodiment the COC-containing layer also forms an intermediatelayer of the film having more than one layer. Other embodiments withCOC-containing intermediate layers have a five-layer structure and haveCOC-containing intermediate layers on both sides, besides theCOC-containing base layer. In another embodiment the COC-containinglayer may form (an) outer layer(s) on one or both sides of the baselayer or intermediate layer, additionally to the base layer. For thepurposes of the present invention, the base layer is that layer whichmakes up from more than about 50 to 100%, preferably from about 70 to90%, of the total film thickness. The outer layer is the layer whichforms the outermost layer of the film.

Each embodiment of the invention is a non-transparent, opaque film. Forthe purposes of the present invention, non-transparent films are filmswhose light transmittance as measured by ASTM-D 1003-77 is less thanabout 95%, preferably less than about 75%.

The COC-containing layer (the base layer) of the novel film comprises apolyester, preferably a polyester homopolymer, a COC, and also, ifdesired, other additives, in each case in effective amounts. This layergenerally comprises at least about 20% by weight, preferably from about40 to 96% by weight, in particular from about 70 to 96% by weight, ofpolyester, based on the weight of the layer.

The base layer of the film comprises a thermoplastic polyester.Polyesters suitable here are those made from ethylene glycol andterephthalic acid (e.g., polyethylene terephthalate, PET), from ethyleneglycol and naphthalene-2,6-dicarboxylic acid (e.g., polyethylene 2,6-naphthalate, PEN), from 1,4-bishydroxymethylcyclo-hexane andterephthalic acid (e.g., poly-1,4-cyclohexanedimethylene terephthalate,PCDT) or else made from ethylene glycol, naphthalene-2,6-dicarboxylicacid and biphenyl4,4′-dicarboxylic acid (e.g., polyethyl-ene2,6-naphthalate bibenzoate, PENBB). Particular preference is given topolyesters which are composed of at least 90 mol %, preferably at least95 mol %, of ethylene glycol units and terephthalic acid units orethylene glycol units and naphthalene-2,6-dicarboxylic acid units. Theremaining monomer units are derived from other aliphatic, cycloaliphaticor aromatic diols and, respectively, dicarboxylic acids, as may also bepresent in layer A (A=outer layer 1) or in layer C (C=outer layer 2) ofa multilayered ABC (B=base layer) film.

Examples of suitable aliphatic diols are diethylene glycol, triethyleneglycol, aliphatic glycols of the formula HO—(CH₂)_(n)—OH, where n is aninteger from 3 to 6 (in particular 1,3-propanediol, 1,4-butanediol,1,5-pentanediol and 1,6-hexanediol) or branched aliphatic glycols havingup to 6 carbon atoms. Among the cycloaliphatic diols, mention should bemade of cyclohexanediols (in particular 1,4-cyclohex-anediol). Othersuitable aromatic diols are those, for example, of the formulaHO—C₆H₄—X—C₆H₄—OH where X is —CH₂—, —C(CH₃)₂—, —C(CF₃)₂—, —O—, —S— or—SO2—. Bisphenols of the formula HO—C₆H₄—C₆H₄—OH are also highlysuitable.

Preferred aromatic dicarboxylic acids are benzenedicarboxylic acids,naphthalenedicarboxylic acids (such as naphthalene-1,4- or-1,6-dicarboxylic acid), biphenyl-x,x′-dicarboxylic acids (in particularbiphenyl -4,4′-dicarboxylic acid), diphenylacetylene-x,x′-dicarboxylicacids (in particular diphenylacetylene-4,4′-dicarboxylic acid) andstilbene-x,x′-dicarboxylic acids. Among the cycloaliphatic dicarboxylicacids mention should be made of cyclo-hexanedicarboxylic acids (inparticular cyclohexane-1,4-dicarboxylic acid). Among the aliphaticdicarboxylic acids the (C₃-C₁₉)-alkanedioic acids are particularlysuitable, where the alkane moiety may be straight-chain or branched.

The polyesters may, for example, be prepared by the transesterificationprocess. The starting materials here are dicarboxylic esters and diols,and these are reacted using the usual transesterification catalysts,such as salts of zinc, calcium, lithium, magnesium or manganese. Theintermediates are then polycondensed in the presence of typicalpolycondensation catalysts, such as antimony trioxide or titanium salts.They may equally well be prepared by the direct esterification processin the presence of polycondensation catalysts, starting directly fromthe dicarboxylic acids and the diols.

According to the invention the COC-containing layer or, in the case ofsingle-layer embodiments, the film, comprises an amount of not less thanabout 4.0% by weight, preferably from about 5 to 50% by weight andparticularly preferably about 6 to 40% by weight, of a cycloolefincopolymer (COC), based on the weight of the layer or, in the case ofsingle-layer embodiments, based on the weight of the film. It issignificant for the present invention that the cycloolefin copolymer(COC) is not compatible with polyethylene terephthalate and does notform a homogeneous mixture with the same.

Cycloolefin polymers are homopolymers or copolymers, which containpolymerized cycloolefin units and, if desired, acyclic olefins ascomonomers. Cycloolefin polymers suitable for the present inventioncontain from about 0.1 to 100% by weight, preferably from about 10 to99% by weight, particularly preferably from about 50 to 95% by weight,of polymerized cycloolefin units in each case based on the total weightof the cycloolefin polymer. Particular preference is given to polymerswhich have been built up using the monomers comprising the cyclicolefins of the formulae I, II, III, IV, V or VI:

R¹, R², R³, R⁴, R⁵, R⁶, R⁷ and R⁸ in these formulae may be identical ordifferent and may be a hydrogen atom or a C₁-C₃₀-hydrocarbon radical, ortwo or more of the radicals R¹ to R⁸ may be bonded cyclically, and thesame radicals in the different formulae may have the same or differentmeaning. Examples of C₁-C₃₀-hydrocarbon radicals are linear or branchedC₁-C₈-alkyl radicals, C₆-C₁₈-aryl radicals, C₇-C₂₀-alkylenearyl radicalsand cyclic C₃-C₂₀-alkyl radicals and acyclic C₂-C₂₀-alkenyl radicals.

If desired, the cycloolefin polymers may contain from 0 to 45% byweight, based on the total weight of the cycloolefin polymer, ofpolymerized units of at least one monocyclic olefin of the formula VII:

wherein n is a number from 2 to 10.

If desired, the cycloolefin polymers may contain from 0 to 99% byweight, based on the total weight of the cycloolefin polymer, ofpolymerized units of an acyclic olefin of the formula VIII:

R⁹, R¹⁰, R¹² and R here may be identical or different and may be ahydrogen atom or a C₁-C₁₀-hydrocarbon radical, e.g. a C₁-C₈-alkylradical or a C₆-C₁₄-alkyl radical.

Other polymers suitable in principle are cycloolefin polymers, which areobtained by ring-opening polymerization of at least one of the monomersof the formulae I to VI, followed by hydrogenation.

Cycloolefin homopolymers have a structure composed of one monomer of theformulae I to VI. These cycloolefin polymers are less suitable for thepurposes of the present invention. Polymers suitable for the purposes ofthe present invention are cycloolefin copolymers (COC) which comprise atleast one cycloolefin of the formulae I to VI and at least one acyclicolefins of the formula VIII as comonomers. These cycloolefin copolymers(COC), which can be used according to the invention, are termed COCsabove and below. Acyclic olefins preferred here are those which havefrom 2 to 20 carbon atoms, in particular unbranched acyclic olefinshaving from 2 to 10 carbon atoms, for example ethylene, propylene and/orbutylene. The proportion of polymerized units of acyclic olefins of theformula VIII is up to 99% by weight, preferably from about 5 to 80% byweight, particularly preferably from about 10 to 60% by weight, based onthe total weight of the respective cycloolefin copolymer.

Among the cycloolefin copolymers described above those which areparticularly preferred contain polymerized units of polycyclic olefinshaving a fundamental nor-bornene structure, particularly preferablynorbornene or tetracyclododecene. Particular preference is also given tocycloolefin copolymers (COCs) which contain polymerized units of acyclicolefins, in particular ethylene. Particular preference is, again, givento norbornene-ethylene copolymers and tetracyclododecene-ethylenecopolymers which in each case contain from about 5 to 80%, preferablyfrom about 10 to 60% by weight of acrylic olefin (based on the weight ofthe copolymer). Particularly suitable cycloolefin copolymers (COC) arepolynorborene, polydimethyloctahydronaphthalene, polycyclopentene, andpoly (5-methyl) norborene.

The cycloolefin polymers generically described above generally haveglass transition temperatures of from −20 to 400° C. Cycloolefincopolymers (COCs) which can be used for the invention have a glasstransition temperature above about 70° C., preferably above about 90° C.and in particular above about 110° C. The viscosity number (decalin,135° C., DIN 53 728) is advantageously from about 0.1 to 200 ml/g,preferably from about 50 to 150 ml/g.

The cycloolefin copolymers (COCs) are prepared by heterogeneous orhomogeneous catalysis with organometallic compounds, as described in awide variety of documents. Suitable catalyst systems based on mixedcatalysts made from titanium compounds and, respectively, vanadiumcompounds in conjunction with aluminum organyl compounds are describedin DD 109 224, DD 237 070 and EP-A-0 156 464. EP-A-0 283 164, EP-A-0 407870, EP-A-0 485 893 and EP-A-0 503 422 describe the preparation ofcycloolefin copolymers (COCs) with catalysts based on solublemetallocene complexes. The preparation processes described in theabovementioned specifications for cycloolefin polymers are expresslyincorporated herein by way of reference.

The cycloolefin copolymers are incorporated into the film either in theform of pure granules or in the form of granulated concentrate(masterbatch), by premixing the polyester granules or polyester powderwith the cycloolefin copolymer (COC) or, respectively, with thecycloolefin copolymer (COC) masterbatch, followed by feeding to anextruder. In the extruder the mixing of the components continues andthey are heated to the processing temperature. It is advantageous herefor the novel process if the extrusion temperature is above the glasstransition temperature T_(g) of the cycloolefin copolymer (COC),generally above the glass transition temperature of the cycloolefincopolymer (COC) by at least about 5° C., preferably by from about 10 to180° C., in particular by from about 15 to 150° C.

For the intermediate layers and for the outer layers it is possible inprinciple to use the polymers used for the base layer. Besides these,other materials may also be present in the outer layers. The outerlayers are then preferably composed of a mixture of polymers, of acopolymer or of a homopolymer, which comprise ethylene 2,6-naphthalateunits and ethylene tere-phthalate units. Up to about 30 mol % of thepolymers may be composed of other comonomers (e.g. ethylene isophthalateunits).

The base layer and the other layers may additionally compriseconventional additives, such as stabilizers, antiblocking agents andother fillers. They are advantageously added to the polymer or,respectively, to the polymer mixture straightaway prior to melting.Examples of stabilizers used are phosphorus compounds, such asphosphoric acid or phosphoric esters.

Typical antiblocking agents (in this context also termed pigments) areinorganic and/or organic particles, such as calcium carbonate, amorphoussilica, talc, magnesium carbonate, barium carbonate, calcium sulfate,barium sulfate, lithium phosphate, calcium phosphate, magnesiumphosphate, aluminum oxide, lithium fluoride, the calcium, barium, zincor manganese salts of the dicarboxylic acids used, carbon black,titanium dioxide, kaolin or crosslinked polymer particles, e.g.polystyrene particles or acrylate particles.

The additives selected may also comprise mixtures of two or moredifferent antiblocking agents or mixtures of antiblocking agents of thesame composition but different particle sizes. The particles may beadded to the polymers of the individual layers of the film in therespective advantageous concentrations, e.g. as a glycolic dispersionduring the polycondensation or via masterbatches during extrusion.Pigment concentrations which have proven particularly suitable are fromabout 0 to 25% by weight (based on the weight of the respective layer).More preferably, the concentrations of pigments, when used, are about0.5 to 25% by weight (based on the weight of the respective layer).EP-A-0 602 964, for example, describes the antiblocking agents indetail.

To improve the whiteness of the film the base layer or the otheradditional layers may comprise further pigmentation, which are alsoknown as white fillers. It has proven particularly advantageous here forthe additional materials added to be barium sulfate with a particle sizeof from about 0.3 to 0.8 μm, preferably from about 0.4 to 0.7 μm, ortitanium dioxide with a particle size of from about 0.05 to 0.3 μm. Thisgives the film a brilliant white appearance. The concentration of bariumsulfate particles is from about 1 to 25% by weight, preferably from morethan about 1 to 20% by weight, and very preferably from about 1 to 15%by weight.

The total thickness of the film may vary within wide limits and dependson the application envisaged. The preferred embodiments of the novelfilm have total thicknesses of from about 4 to 400 μm, preferably fromabout 8 to 300 μm, particularly preferably from about 10 to 300 μm. Thethickness of any intermediate layer(s) present is/are, in each caseindependently of one another, from about 0.5 to 15 μm, preferably fromabout 1 to 10 μm, in particular from about 1 to 8 μm. All the valuesgiven are based on one intermediate layer. The thickness of the outerlayer(s) is selected independently of the other layers and is preferablywithin the range from about 0.1 to 10 μm, in particular from about 0.2to 5 μm, preferably from about 0.3 to 2 μm, and outer layers applied onboth sides may be identical or different in terms of their thicknessesand compositions. The thickness of the base layer is therefore given bythe difference between the total thickness of the film and the thicknessof the outer and intermediate layer(s) applied, and, similarly to thetotal thickness, may therefore vary within wide limits.

The invention also provides a process for producing the novel polyesterfilm by the extrusion or coextrusion process known per se.

For the purposes of the present invention the procedure is toextrude/coextrude, through a flat-film die, the melt(s) corresponding tothe single-layer film or to the individual layers of the film, to drawoff the resultant film on one or more roll(s) to solidify the film, thento stretch (orient) the film biaxially, to heat-set the biaxiallystretched film, and, if desired, to corona- or flame-treat the surfacelayer intended for treatment.

The biaxial orientation procedure is generally carried out in sequence.For this, longitudinal orientation (i.e. in the machine direction=MD) ispreferably the first procedure, followed by transverse orientation (i.e.perpendicularly to the machine direction=TD). This gives an orientationof the molecular chains. The longitudinal orientation procedurepreferably takes place with the aid of two rolls running at differentspeeds corresponding to the stretching ratio desired. For the transversestretching procedure an appropriate tenter frame is generally utilized.

The use of simultaneous orientation procedures for the novel film inboth directions (MD and TD) with the aid of a suitable tenter frame hasnot proven useful. Specifically, this stretching gives a film which hasinsufficient whiteness and insufficient opacity.

The temperature at which the orientation procedure is carried out mayvary within a relatively large range and depends on the film propertiesdesired. The longitudinal stretching is generally carried out at fromabout 80 to 130° C. and the transverse stretching at from about 90 to150° C. The longitudinal stretching ratio is generally within the rangefrom about 2.5:1 to 6:1, preferably from about 3:1 to 5.5:1. Thetransverse stretching ratio is generally within the range from about3.0:1 to 5.0:1, preferably from about 3.5:1 to 4.5:1.

In the heat-setting which follows, the film is held at a temperature offrom about 150 to 250° C. for from about 0.1 to 10 s. The film is thenwound up in the usual manner.

To establish other desired properties, the film may be chemicallytreated or else corona- or, respectively, flame-treated. The intensityof treatment is selected such that the surface tension of the film isgenerally above about 45 mN/m.

To establish other properties, the film may also be coated. Typicalcoatings have adhesion-promoting, antistatic, slip-improving or releaseaction. It is clear that these additional coatings may be applied to thefilm by in-line coating using aqueous dispersions, prior to thetransverse stretching procedure.

The particular advantage of the novel film is its high whiteness andhigh opacity. Surprisingly, the gloss of the film was also very high.The whiteness of the film is above about 70%, preferably above about 75%and particularly preferably about 80%. The opacity of the novel film isabove about 55%, preferably above about 60% and particularly preferablyabove about 65%. The gloss of the novel film is above about 80,preferably above about 90 and particularly preferably above about 100.

Another particular advantage of the invention is that regeneratedmaterial produced directly during the production process can be reusedat a concentration of from about 10 to 70% by weight, based on the totalweight of the film, without any significant negative effect on thephysical properties of the film. In particular, the regenerated material(composed essentially of polyester and cycloolefin copolymers (COC))does not give undefined changes in the color of the film, as is the caseof the films of the prior art.

A further advantage of the invention is that the production costs of thenovel film are comparable with those of conventional transparent filmsof the prior art. The other properties of the novel film relevant to itsprocessing and use remain essentially unchanged or are even improved.

The film has excellent suitability for packing foods or other consumableitems which are sensitive to light and/or to air. It is also highlysuitable for use in the industrial sector, e.g. for producinghot-stamping foils or as a label film. Besides this, the film is, ofcourse, particularly suitable for image-recording papers, printedsheets, magnetic recording cards, to name just a few possibleapplications.

The processing performance and winding performance of the film, inparticular on high-speed machines (winders, metallizers, printingmachines and laminating machines) is exceptionally good. A measure ofprocessing performance is the coefficient of friction of the film, whichis below about 0.6. A decisive factor affecting winding performance,besides a good thickness profile, excellent layflat and a lowcoefficient of friction, is the roughness of the film. It has becomeapparent that the winding of the novel film is particularly good if theaverage roughness is within a range from about 50 to 250 nm while theother properties are complied with. The roughness may be varied withinthe stated range by, inter alia, varying the COC concentration and theprocess parameters in the production process. The table below (Table 1)summarizes the most important film properties according to theinvention.

TABLE 1 Range according Partic- to the ularly invention Preferredpreferred Unit Test method Composition Concentration 4-60 5-50 6-40% %of cycloolefin copolymer (COC) in base layer Glass 70-270 90-250 110-220° C. DIN 73 765 transition temperatures of cycloolefin copolymer (COC)Film properties Whiteness >70 >75 >80 % Berger Opacity >55 >60 >65 % DIN53 146 Coefficient <0.6 <0.55 <0.5 DIN 53 375 of Friction, COFGloss >80 >90 >100 DIN 67 530 Average 50-250 60-230 70-200 nm DIN 4768,roughness, cut-off of R_(a) 0.25 mm

The following parameters were used to describe the polymers and thefilms:

SV (Solution Viscosity)

To determine the SV, a specimen of polyester was dissolved in a solvent(dichloroacetic acid). The viscosity of this solution, and also theviscosity of the pure solvent, are measured in an Ubbelohde viscometer.The quotient is calculated from the two values, 1.000 is deducted fromthis and the resultant value multiplied by 1000. This gives the SV(solution viscosity).

Coefficient of Friction

The coefficient of friction, COF, is determined according to DIN 53 375.The coefficient of sliding friction was measured 14 days afterproduction of the film.

Surface Tension

The surface tension was determined by the ink method (DIN 53 364).

Roughness

The roughness, R_(a), of the film was determined according to DIN 4768with a cut-off of 0.25 mm.

Whiteness and Opacity

The whiteness and opacity were determined with the aid of a Zeiss,Oberkochem (DE) “ELREPHO” reflectance photometer, standard illuminant C,2° C. normal observer. Opacity is determined according to DIN 53 146.Whiteness is defined as W=RY+3RZ−3RX, wherein W=whiteness and RY, RZ andRX=relevant reflection factors when the Y, Z and X color-measurementfilter is used. The white standard used was a barium sulfate pressing(DIN 5033, Part 9). A detailed description is given, for example, inHansl Loos “Farbmessung”, Verlag Beruf und Schule, Itzehoe (1989).

Light Transmittance

Light transmittance is measured using a method based on ASTM-D 1033-77.

Gloss

Gloss was determined according to DIN 67 530. The reflectance wasmeasured as an optical value characteristic of a film surface. Based onthe standards, ASTM-D 523-78 and ISO 2813, the angle of incidence wasset at 60°. A beam of light hits the flat test surface at the set angleof incidence and is reflected and/or scattered by this surface. Aproportional electrical variable is displayed representing light beamshitting the photoelectronic detector. The value measured isdimensionless and must be stated together with the angle of incidence.

Glass Transition Temperature

The glass transition temperature, Tg, was determined using filmspecimens with the aid of DSC (differential scanning calorimetry) (DIN73 765). A DuPont DSC 1090 was used. The heating rate was 20° K./min andthe specimen weight was about 12 mg. The glass transition transition,Tg, was determined in the first heating procedure. Many of the specimensshowed an enthalpy relaxation (a peak) at the beginning of the step-likeglass transition. The temperature taken as Tg was that at which thestep-like change in heat capacity—without reference to the peak-shapedenthalpy relaxation—achieved half of its height in the first heatingprocedure. In all cases, there was only a single glass transitionobserved in the thermogram in the first heating procedure.

EXAMPLE 1

Chips of polyethylene terephthalate (prepared by the transesterificationprocess using a Mn transesterification catalyst, Mn concentration: 100ppm) were dried at 150° C. to a residual moisture below 100 ppm and fedto the extruder for the base layer B. Alongside this, chips of ®Topas6015 cycloolefin copolymer (COC) from Ticona (COC composed of2-norbornene and ethylene, see also W. Hatke: Folien aus COC [COCFilms], Kunststoffe 87 (1997) 1, pp. 58-62) with a glass transitiontemperature Tg of about 160° C. were also fed to the extruder for thebase layer B. The proportional amount of the cycloolefin copolymer (COC)in the entire film was 10% by weight.

Extrusion followed by a stepwise orientation procedure in longitudinaland transverse directions was used to produce a white, opaquesingle-layer film with a total thickness of 23 μm.

Base layer B was a mixture of:

90.0% by weight of polyethylene terephthalate homopolymer with an SV of800 and 10.0% by weight of Topas 6015 cycloolefin copolymer (COC) fromTicona

The process conditions in the individual steps were:

Extrusion: Temperatures of base layer: 280° C. Temperature of thetake-off roll: 30° C. Longitudinal stretching: Temperature: 80-125° C.Longitudinal stretching ratio: 4.2 Transverse stretching Temperature:80-135° C. Transverse stretching ratio: 4.0 Setting: Temperature: 230°C. Duration: 3 s

The film had the good properties required and the desired handling andprocessing performance. The properties achieved in films produced inthis way are given in Table 2.

EXAMPLE 2

Unlike in Example 1, 50% by weight of regenerated material was now addedto the base. The concentration of the cycloolefin copolymer (COC) in thefilm thus produced was again 10% by weight. The process parameters wereunchanged from Example 1. A visual observation was made of any yellowdiscoloration of the film. Table 2 shows that hardly any yellowdiscoloration could be seen.

Base layer B was a mixture of:

45.0% by weight of polyethylene terephthalate homopolymer with an SV of800, 50.0% by weight of regenerated material (90% by weight ofpolyester + 10% by weight of Topas 6015) and  5.0% by weight of Topas6015 cycloolefin copolymer (COC) from Ticona

EXAMPLE 3

Unlike in Example 1, the film now produced had a thickness of 96 μm. Theconcentration of the cycloolefin copolymer (COC) in the film was 8% byweight. The process parameters were unchanged from Example 1. A visualobservation was made of any yellow discoloration of the film. Table 2shows that no yellow discoloration of the film could be seen.

Base layer B was a mixture of:

92.0% by weight of polyethylene terephthalate homopolymer with an SV of800 and  8.0% by weight of Topas 6015 cycloolefin copolymer (COC) fromTicona

EXAMPLE 4

Unlike in Example 3, 50% by weight of regenerated material was now addedto the base. The concentration of the cycloolefin copolymer (COC) in thefilm was again 8% by weight. The process parameters remained unchangedfrom Example 1. A visual observation was made of any yellowdiscoloration of the film. Table 2 shows that hardly any yellowdiscoloration could be seen.

Base layer B was a mixture of:

56.0% by weight of polyethylene terephthalate homopolymer with an SV of800 and 50.0% by weight of the regenerated material from the sameprocess (90% by weight of polyester + 10% by weight of Topas 6015) and 4.0% by weight of Topas 6015 cycloolefin copolymer (COC) from Ticona

COMPARATIVE EXAMPLE 1

Example 1 from DE-A 2 353 347 was repeated. The example was modified byadditionally including 50% by weight of regenerated material in theprocess. Table 2 shows that a marked yellow discoloration of the filmcould be seen. In addition, the roughness of the film is much too highfor many applications and its gloss is too low for many applications.This is very probably attributable to the use of other polymericadditives.

Base layer B was a mixture of:

47.5% by weight of polyethylene terephthalate homopolymer with an SV of800 and 50.0% by weight of regenerated material from the same process(95% by weight of polyester + 5% by weight of polypropylene) and  2.5%by weight of polypropylene

COMPARATIVE EXAMPLE 2

Example 1 from EP-A 0 300 060 was repeated. The example was modified byadditionally including 50% by weight of regenerated material in theprocess. Table 2 shows that a marked yellow discoloration of the filmcould be seen. In addition, the roughness of the film is much too highfor many applications and its gloss is too low for many applications.This is very probably attributable to the use of other polymericadditives.

Base layer B was a mixture of:

45.0% by weight of polyethylene terephthalate homopolymer with an SV of800 and 50.0% by weight of regenerated material from the same process(95% by weight of polyester + 5% by weight of polypropylene) and  5.0%by weight of polypropylene

COMPARATIVE EXAMPLE 3

Example 1 from EP-A 0 360 201 was repeated. The example was modified byadditionally including 50% by weight of regenerated material in theprocess. Table 2 shows that a marked yellow discoloration of the filmcould be seen. In addition, the roughness of the film is much too highfor many applications and its gloss is too low for many applications.This is very probably attributable to the use of other polymericadditives.

Base layer B was a mixture of:

40.0% by weight of polyethylene terephthalate homopolymer with an SV of800 and 50.0% by weight of regenerated material from the same process(95% by weight of polyester + 5% by weight of polypropylene) and 10.0%by weight of polypropylene

COMPARATIVE EXAMPLE 4

Example 1 from DE-A 195 40 277 was repeated. The example was modified byadditionally including 50% by weight of regenerated material in theprocess. Table 2 shows that a marked yellow discoloration of the filmcould be seen. In addition, the roughness of the film is much too highfor many applications and its gloss is too low for many applications.This is very probably attributable to the use of other polymericadditives.

Base layer B was a mixture of:

43.5% by weight of polyethylene terephthalate homopolymer with an SV of800 and 50.0% by weight of regenerated material from the same process(95% by weight of polyester + 5% by weight of polystyrene) and  6.5% byweight of polystyrene

TABLE 2 Coefficient Additive of Average concen- Glass friction roughnesstration transition Evaluation COF R_(a) Film in temperature of Side A nmthickness Layer Added to base layer of additive Whiteness Opacity filmagainst Side Side Example μm structure polyester % ° C. % % yellownessGloss Side C A C E 1  23 B COC 10 170 75 75 ++ 115 0.52 120 120 E 2  23B COC 10 170 76 80 + 120 0.50 110 110 E 3  96 B COC  8 170 85 85 ++ 1250.42 100 100 E 4  96 B COC  8 170 86 90 + 130 0.35  98  98 CE 1 155 BPolypropylene  5 −10 80 70 −  46 0.45 410 410 CE 2 100 B Polypropylene10 −10 88 80 −  57 0.45 180 180 CE 3 100 ABA Polypropylene 20 −10 92 89−  54 0.25 370 370 CE 4 125 B Polystyrene 13 100 82 82 −  51 0.35 480480 Key to yellowness in films produced: ++ no yellowing detectable +slight yellow coloration detectable − marked yellow colorationdetectable

What is claimed is:
 1. A white, biaxially oriented polyester filmcomprising at least a base layer and optionally one or more otherlayers, wherein at least the base layer comprises a polyester and acycloolefin polymer, wherein the cycloolefin polymer is anorbornene-ethylene copolymer or a tetracyclododecene-ethylenecopolymer, wherein the cycloolefin polymer is present at a concentrationranging from about 4 to about 60% by weight, based on this layer, wherethe glass transition temperature of the cycloolefin polymer is above110° C., wherein the film has a gloss of about 80 or above and anopacity of about 55% or above, contains from about 10% to about 70%recycled material, and is oriented using a transverse stretching ratioof from about 3.5:1 to about 4.5:1.
 2. A polyester film according toclaim 1, wherein the base layer comprises a cycloolefin polymer at aconcentration ranging from about 5 to 50% by weight, based on thislayer.
 3. A polyester film according to claim 1, wherein the base layercomprises a cycloolefin polymer at a concentration ranging from about 6to 40% by weight, based on this layer.
 4. A polyester film according toclaim 1, wherein the cycloolefin polymer has a glass transitiontemperature ranging up to 220° C.
 5. A polyester film according to claim1, wherein the film has a whiteness above about 70%.
 6. A polyester filmaccording to claim 1, wherein the film has a whiteness above about 75%.7. A polyester film according to claim 1, wherein the film has awhiteness above about 80%.
 8. A polyester film according to claim 1,wherein the film has an opacity above about 60%.
 9. A polyester filmaccording to claim 1, wherein the film has an opacity above about 65%.10. A polyester film according to claim 1, wherein the film has a glossabove about
 90. 11. A polyester film according to claim 1, wherein thefilm has a gloss above about
 100. 12. A polyester film according toclaim 1, wherein each layer comprises from about 0 to 25% by weight,based in each case on the weight of the layer, of anti-blocking agents,vacuole initiators, white fillers or mixtures thereof.
 13. A polyesterfilm according to claim 1, wherein at least one outer layer has beenarranged on the cycloolefin polymer-containing base layer.
 14. Apolyester film according to claim 13, further comprising one or moreintermediate layers, wherein an intermediate layer has been arrangedbetween the cycloolefin polymer-containing base layer and an outerlayer.
 15. A polyester film according to claim 1, wherein the film is asingle-layer film composed of the cycloolefin polymer-containing baselayer.
 16. A method for packaging foodstuffs and other consumable items,comprising packaging said foodstuffs and other consumable items in a byfilm as claimed in claim
 1. 17. A method for recording information,comprising recording said information on a film as claimed in claim 1.18. A process for producing a white, biaxially oriented polyester filmaccording to claim 1, comprising the steps: (A) extruding a melt foreach layer through a flat-film die; (B) drawing off the resultant filmonto at least one roller; (C) biaxially stretching the resultantsolidified film; and (D) heat setting the biaxially stretched film. 19.The process of claim 18, wherein the biaxially stretched film ischemical-, corona- or flame-treated.
 20. The process of claim 18,wherein the biaxial stretching is conducted by first stretching thesolidified film in the longitudinal direction and then by stretching inthe transverse direction.